Abstract: To elucidate the influence of equivalence ratio (φ) on the detonation behaviour of CH₄-O₂ mixtures under low-temperature conditions, low-temperature detonation experiments were conducted within the temperature range 203.15–283.15 K. Four premixed mixtures with φ = 1.0, 1.33, 1.45 and 1.6 were ignited in a smooth square tube, and the peak pressure as well as the propagation velocity of the detonation wave were recorded. The results show that stable detonation can be established for all equivalence ratios at low temperature and high pressure, whereas no stable detonation is observed at low temperature and low pressure (p = 5 kPa). Concerning peak pressure, only considering the cases of successful ignition, the overpressure only increases with φ at lower-temperatures (T = 203.15–243.15 K); while a decreasing trend is observed at other temperatures. Compared with the chemical equivalence ratios, the propagation velocity of the high equivalence ratio was reduced, and the velocity loss is significantly increased. The propagation velocity can only reached about 0.9 vCJ after stable propagation, with a maximum deficit of 3.46 %. However, the chemical equivalence ratio fails to maintain a stable detonation near the tube exit, whereas the higher equivalence ratios retain their propagation speed and remain stable. Kinetic analysis was performed using CHEMKIN to elucidate the combustion mechanism. It was revealed that the ignition delay increases by up to 2.76 × 10⁻⁴ s as the equivalence ratio increases, and the elementary reaction R73 supersedes R45 in importance. The effect of equivalence ratio on OH radical sensitivity was more significant in the low-temperature environment. Both experimental and numerical results demonstrate that high equivalence ratios can better compensate for the detonation limitations imposed by low temperatures, facilitating the formation of stable detonation. The present study provides fundamental guidance for improving the propulsive performance of detonation engines operating with methane–oxygen propellants under low-temperature conditions.